1. Field of the Invention
The present invention relates to a frequency conversion circuit for a color television channel whose signal is received as a radio-frequency antenna or cable signal and which contains a video channel and at least one sound channel separated in frequency from the video channel.
2. Description of the Related Art
The video channel in a color television channel contains the composite color signal of the respective color television standard, e.g., PAL or NTSC. If two sound channels are present, the two channels contain the frequency-modulated television stereo sound according to any of the European stereo standards, while in the U.S.A., for example, the stereophonic signal may be contained in only one sound channel.
Frequency conversion in color television receivers is commonly performed by means of a tuner, which converts the color television channel to be received to a fixed intermediate-frequency band of about 33 MHz to 40 MHz, from which the various signal components are converted to the baseband.
Besides this conventional IF conversion, there is a method which is referred to as low-IF or zero-IF conversion, wherein the radio-frequency signal is converted to the baseband or to a very low frequency band directly, i.e., without IF conversion. The radio-frequency signal is converted by means of a quadrature signal pair having a fixed frequency in the range of the radio-frequency signal, so that the mixer signal is a quadrature signal pair at the low frequency. From this quadrature signal pair, the desired sideband can be isolated at baseband by various methods used in single-sideband systems, such as the phasing method or the "third" method.
This separation, as is well known, is achieved by adding or subtracting the modified quadrature signal pair by means of a suitable combinatorial circuit. In the phasing method, one signal of the quadrature signal pair is shifted in phase by 90.degree. before this coupling circuit, and in the third method, the quadrature signal pair is subjected to a second quadrature conversion. To separate adjacent signals, only low-pass filters to which the combined signal is applied are necessary in both cases.
In the case of television signals, such a direct frequency conversion is critical, because asymmetries and phase errors in the quadrature-signal path produce moire on the television screen. This is mainly due to the high picture-carrier content in the RF signal, because this content is usually not fully suppressed during the single-sideband conversion and, thus, becomes visible during reproduction as a fixed-frequency spurious signal within the video signal. A certain remedy is provided by digital signal processing, in which case the digitization should take place as early as possible in the signal path, because changes in the quadrature-signal paths due to aging, temperature variations, settings, and interference can then be largely avoided Furthermore, precisely defined filter circuits can be implemented digitally, which would hardly be realizable in analog form without alignment.
Such a frequency conversion circuit for color television signals, including the associated sound channels, is described in U.S. Pat. No 4,710,814. FIG. 2 of that patent specification shows a low-IF converter whose quadrature output signals are digitized by means of two analog-to-digital converters and fed to the video-signal-processing circuit, which delivers the digitized composite color signal. The digitized quadrature signal pair is also fed through a combination stage to the sound-processing stage, which converts the two sound channels to a low frequency and separates them from each other. The frequency of the local oscillator lies approximately in the middle between the picture carrier and the chrominance subcarrier, so that in the video signal a visible spurious signal is formed at 4.4 MHz by the residual picture carrier.